In order to detect vertical tear of a conveyor belt (cracks continuous in the belt longitudinal direction), a conveyor belt in which loop coils are embedded is known (see Japan Unexamined Patent Publication Nos. 2014-31241 and 2015-71493). In such a conveyor belt, a sensor arranged adjacent to the conveyor belt senses the induced current occurring in loop coils passing near the sensor. If a sharp object or the like to be conveyed pierces the conveyor belt and a vertical tear is generated, the loop coils are damaged, so that no induced current is generated in the loop coils. In this case, the sensor does not detect the induced current even though the loop coils have passed through the vicinity. Thus, it is possible to determine whether or not vertical tear has occurred in the conveyor belt depending on whether or not the induced current is detected by the sensor. If it is determined that vertical tear has occurred on the basis of the detection of the sensor, the operation of the conveyor is stopped in order to prevent expansion of the vertical tear.
Local impacts and external forces are exerted on conveyor belts due to objects to be conveyed or the like to be loaded. Even if vertical tear does not occur in the conveyor belt due to the impact or external force, the loop coils may be damaged. If loop coils are damaged in this way, the sensors do not detect the induced current even if the loop coils passing through the vicinity. Thus, if judged based on the detection of the sensor, it will be erroneously recognized that vertical tears occur in the conveyor belt.
As a result of analyzes and tests of the inventors of the present application, it was found that there is a high correlation between the position of the embedding depth of loop coils in the lower cover rubber and the damage state of the loop coils. However, manufacturing many samples in which the position of embedding depth of the loop coils in lower cover rubber is made different and applying an impact to these samples in order to confirm the damage conditions of the loop coils, a great deal of man-hours (time and cost) are required.